Heat treatment of magnesium alloys



Patented Nov.'.21,

I D S TE-s "PATENT OFFICE I by HEAT rassn m r rrrmaunsmm I Robert 8.Bush, Midland,l\lich., assignor to-The Dow Chemical Company,'llflicllaml. Mich., a corporation-oi Michigan i No Ihawing.AppllcationDecember 23, 1943,

7 Serial N0. 515,419

7 Claims. (Cl. 148-413) I This invention relates to the heat-treatmentof magnesium-base alloys.- It particularlyconcerns ,a preliminarysequence for minimizing germination during solution-heat-treatment oimagnesium-base alloy. castings.

During the heat-treatment '1 metals, there is j sometimes observed. thephenomenon of germination, or sporadic uncontrolled grain growth.

(See, for example, the discussion in" The Science.

- of Metals, p. 133 fl., Jeflries and Archer, Mc- Graw-Hill, New York,1924.) Thus,after prolonged solution-heat-treatment 01' certainmasnesium-base alloy castings; local areas of the metal mayexhibit,insteadof the usual large number of small grains, one or a fewrelatively enormous grains having adiameter of a tenth 1 of an inch ormore; single grains more than half anv inch in size are occasionallyseen. Inasmuchas the occurrence of such macro-crystalline areas may soweaken the metal as to render. it unfit ior service, the avoidance ofgermination during heat-treatment of magnesium alloys is of greatpractical importance. I

, According to the present invention, uncontrolled crystal growth duringheat-treatment oi heat-tr'eatable magnesium-base alloys characterizedbya tendency towardgermination may be largely prevented by apre-treatment which comprises heating the metal-at asolution-heattreating temperature for a time sufllcient to'efiectpartial solution of any major second phase present but insumcienttocause appreciable 4 germination, and then coolingthe metal andsubjecting it to an aging temperature for a time sumcient to eifectsubstantial precipitation 01 a major. second phase; This pre-treatment,in-

-volving partial solution and reprecipitationof' a major second phasepresent-in the alloy, appears to alter'the ultimate. structure of themetal. pos- ,sibly by reorienting crystals-or by creating new centers ofnucleation,- so that, excessive grain growth does not occur duringsubsequent'solution-heat-treatment. In so far as known, the process orthe in'vem tion may be successfully applied to articles formed of, anyheat-treatable magnesium-base" alloy =which tends to undergo germinationon prolonged heating. In this connection, however, it

' may be noted thatthe tendency toward unconk trolled; crystal growth'ismost common with cast metal, especially metal which has been locallystained by drastic'chilling in the mold. The

initiate, germination during subsequent heattreatment of the casting.Commercial magnesium alloys containing. from 8 to 14 percent of aluminumas the major alloying element, especially those in which the usual 0.1to 0.2 per centmanganese is absent, and to a lesser extent those Ialloys containin'gup to 4 per cent of zinc, as

' wellas aluminum and the usual amount of manganese,- appear'to beparticularly susceptible to lo sporadic grain growth. For this reason;the pretreatment according to the invention is most commonly used as apreliminary step in the solution he'at-treatmeut of castings poured fromsuperheated magnesium-aluminum-zino alloys of the type mentioned.

Among thecommercial magnesium base alloys Ito-whichthe invention is mostfrequently applied arernow'metal 0., containing from 8.3 to

9.7- per 'cent aluminum, 1.! to 2.3 per cent zinc,

and about 0.1 per cent manganese; Dowmetal G,

containing from 9 to'l1 per cent of aluminum, up to 0.3 'per cent zinc,about 0.1 per cent manganese; and Dowmetal R, containing from 8.3 to 9.!per cent aluminum, about 0.2 per cent 26 manganese, and up to 1.0per-cent zinc, the balance of each alloy being magnesium.

In practicing the invention, the casting or other article to-be treatedis first heated at a solution-heat-treat ing temperature, 1. e. at atemperature well above the temperature limit of solid solubility oi thesubsisting concentration of the major alloying element in magnesium, Inthe case of magnesium-base alloys containing 8' 'to'l4 per cent ofaluminum, the preferred temperature is in' the range of 715 to 800- F.,be-

ing usually 750 to 'l'm F. The-duration of this treatmentshould besuflicient to cause substantial solution of a any second major phasepresent but not so long as to cause germination, with 40 themagnesium-aluminum alloys f mentioned,

' are usually satisfactory, the longer times being' required at thelower temperatures; 21 131 770? F. is a common proce ure. 3 1

The preliminary heating "in a standard furnace provided'with aprotective atmosphere, or in'a molten salt bath. The metal should beraised to therequired temperature as rapidly as possible, but, as inother heat-treatments of magnesium alloys, heating should not be so fastthat the eutectic phase melts and" burns before itgoes into solidsolution. For

example, a heating time of about 2 hours may be necessary to raise amagnesium-aluminum alloy is best accomplished casting from about 640 F.to a treatment temperature of 770 F.

After the brief partial solution-heat-tre'atment just described, themagnesium alloy article is.

cooled rapidly to an aging temperature or below and is then given anaging treatment for a time sufficient to effect substantialprecipitation of a second phase. -Cooling is ordinarily done in stillair, although forced-draft or other means may 'be used. Aging may becarried out in convenhours are effective, the higher temperaturesrequiring the shorter times.

Following the pre-heating and the aging steps, the castings or otherarticles may be given a conventional solution-heat-treatment, e. g. at715 to 800 F. for to hours for magnesiumaluminum alloys, withoutundergoing substantial germination, a result usually not otherwisepossible with alloys having germinative tendencies. The pretreatmentaccording to the invention in general decreases somewhat the timerequired to eflect the conventional solution-heat-treatm'ent.

In particularly diilicult cases of germination, as with castingssubjected to extreme local chilling, it may occasionally be necesary torepeat the partial solution-heat-treating and aging steps of theinvention one or more times in order entirely to prevent germinationduring prolonged solution-heat-treatment.

Regarding the invention from another viewpoint, it may be seen that theprevention of germination is accomplished essentially by interrupting anordinary solution-heat-treatment prior to the occurrence of germinationand subiecting the article to an aging temperature for a time sumcientto effect substantial precipita tion-hardenlng before resuming thesolutionheat-treatment.

The following example will further illustrate the invention but 'is notto be construed as limiting its scope:

Example A considerable number of identical aircraft castings ofintricate designwere pouredfrom a tive, and that the invention isco-extensive in scope with the following claims.

What is claimed is:

1. An improved procedure for producing in an ungerminated conditionsolution-heat-treated articles formed of a heat-treatable magnesiumbasealloy characterized by a tendency to undergo germination on prolongedheating, which comprises pre-heating the article at asolution-heattreating temperature for a period sufllcient to effectpartial solution of a major second phase in the alloy but insufllcientto produce appreciable germination, then subjecting it to an agingtemperature for a time sufficient to eflect substantial precipitation,and thereafter subjecting the article to solution-heat-treatment.

2. An improved procedure for producing in an ungerminated conditionafter solution-heattreatment castings poured from a superheatedheat-treatable magnesium-base alloy containing from 8 to 14 percentaluminum and up to 4 per cent zinc as the major alloying elements,which comprises pre-heating the casting'at a solutionheat-treatingtemperature of at least 715 F. for a period sufficient to eifect partialsolution of any second phase in the alloy but not exceeding about 8hours, then cooling the casting rapidly and subjecting it to an agingtemperature in the range 325 to 700 F. for a time suflicient. to effectsubstantial precipitation, and thereafter subjecting the casting tosolution-heat-treatment at a temperature of at least 715 F.

3. 'A. process according to claim 2 in which the magnesium-base alloycontains from 8.3 to 9.7

per cent aluminum and from 1.7 to 2.3 per cent .ings formed of aheat-treatable magne ium-base alloy characterizedby a tendency "undergogermination on prolonged heating, a pre-treatment for minimizinggermination during subsequent heat-treatment which comprises heating thecasting at a solution-heat-treatlng tempera- .ture for a time sumcientto effect partial solution of any second phase in the alloy butinsuflicient to cause appreciable germination, and then cooling thecasting and subjecting it to an aging temperature for a time sufllcientto effect substantial precipitation.

- 7. An improvement'in'the solution-heat-treat- In contrast, with anotherwise identical series of castings which were pre-heated at'770' F.for

2 hours and aged at 650 F. for 4 hours in accordance with the invention,and were then solution-heat-treated at 770 F.for 18 hours, less than5-.per cent of the castings showed even traces of germination.

It is to be understood that the foregoing description is illustrativerather than strictly limitament of articles formed of a heat-treatablemagnesium-base alloy characterized by a tendency to undergo germinationon prolonged heating which comprises interrupting thesolution-heat-treatment prior to the occurrence. of appreciablegermination and subjecting the article to an aging temperature for atime sumcient to effect substantial precipitation before resuming thesolution-heat treatment.

ROBERT S. BUSK.

